This invention relates in general to welded web material and processes for producing the welded web material.
Thermoplastic webs may be joined by overlapping the edge of one web over another and placing the overlapped or seam portion of the webs over a base beneath a vibrating welding element. The vibrating welding element may be a horn vibrating at an ultrasonic frequency while it is brought into forced contact with one side of the overlapped webs while the other side of the overlapped web seam is supported on an anvil surface. Transfer of vibratory energy from the horn to the web material is effected upon impact of a suitably shaped surface of the horn on the seam of the web material. The vibrating energy causes surface heat to be generated between the overlapping webs along the area of contiguous contact between the webs. The horn normally resonates toward and away from the seam at a frequency of about 16 kHz or higher. The welding may be in the form of a chain of spots along the seam or a continuous weld. The technique of ultrasonic welding of thermoplastic material is well known and illustrated, for example, in U.S. Pat. No. 3,879,256, U.S. Pat. No. 3,939,033, U.S. Pat. No. 3,947,307 and U.S. Pat. No. 3,459,610, all incorporated by reference herein in their entirety.
Although ultrasonic welding can be utilized to join together webs of thermoplastic material, it has been found that the welded seam is the weakest portion of the joined webs and can be, for example, only about one-third as strong as the remainder of the joined webs. Further, the joint formed by ultrasonic welding tends to be more brittle than the remainder of the joined webs. Moreover, coated webs, particularly those having a coating thickness greater than about 15 micrometers, tend to delaminate at the seam when bent around small diameter rods or rollers. Thus, welded thermoplastic webs and techniques for making welded webs exhibit deficiencies such as welded web seams that have less strength and increased brittleness.